1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus, and more particularly relates to an apparatus that folds a sheet bundle and performs bookbinding processing.
2. Description of the Related Art
There is a conventional image forming apparatus (e.g., a copying machine, a laser beam printer, etc.) equipped with a sheet processing apparatus that can appropriately fold respective sheets discharged from the image forming apparatus, or stitch the sheets along their center lines and then fold the stitched sheets for saddle stitch bookbinding.
In the saddle stitch bookbinding processing, if the number of sheets constituting a sheet bundle is large (e.g., 20 or more) and the bundle of sheets is bent for bookbinding, a folded spinal portion of a finished product has a curvature. The sheet bundle finished in this manner is generally insufficient in foldness and may open somewhat even after the sheet bundle is firmly pressed and folded. Therefore, the finished product does not look nice. If numerous sheet bundles are stacked in the vertical direction, their surfaces may not be held horizontally due to irregularity in thickness.
To eliminate such drawbacks, as discussed in U.S. Pat. No. 6,692,208, or US Pub2003/0031532, a conventional sheet processing apparatus includes a pressing roller that can travel along a folded spinal portion (i.e., a bent portion) of a folded sheet bundle while pressing the folded spinal portion to corner (smooth) a curvature of the folded spinal portion.
FIGS. 27A to 27C illustrate an example configuration of a conventional sheet processing apparatus. When the sheet processing apparatus performs processing for cornering a folded spinal portion of a sheet bundle as a deforming process, a pair of belt conveyors 16 and 17 conveys a folded sheet bundle SA until the folded spinal portion collides against a positioning member 15 as illustrated in FIG. 27A. After the folded spinal portion abuts against the positioning member 15, the belt conveyors 16 and 17 continuously rotate a predetermined amount to further convey the sheet bundle SA while causing slip on their surfaces. This is effective to correct a skew of the sheet bundle SA and accurately adjust the position of the sheet bundle SA.
Next, as illustrated in FIG. 27B, the sheet bundle SA is held between a pair of grip portions 12 and 13 with its folded spinal portion in a protruded state. Namely, the grip portions 12 and 13 cooperatively fix the sheet bundle SA at a position adjacent to the folded spinal portion. The positioning member 15 moves to a retreat position. Then, as illustrated in FIGS. 28A and 28B, a pressing roller 14 travels in a direction indicated by an arrow while pressing a folded spinal portion of the curved sheet bundle SA that protrudes from the grip portions 12 and 13. Thus, the folded spinal portion of the curved sheet bundle SA can be cornered and smoothed. Then, as illustrated in FIG. 27C, the belt conveyors 16 and 17 convey and discharge the cornered (i.e., smoothed) sheet bundle SA to a sheet discharge tray 18.
In the above-described conventional sheet processing apparatus, the pressing roller 14 is located at a predetermined retreat position before performing the processing for cornering a folded spinal portion so as not to interfere with a sheet bundle being conveyed. The retreat position of the pressing roller 14 is moved from a conveyance path along which the sheet bundle is conveyed in a direction perpendicular to the sheet bundle conveyance direction (hereinafter, referred to as a “width direction”). When the pressing roller 14 starts cornering the folded spinal portion of the sheet bundle SA positioned by the positioning member 15, the pressing roller 14 moves from the retreat position (i.e., one end side) to the other end side of the folded spinal portion while pressing the folded spinal portion.
However, when the pressing roller 14 moves from the retreat position to the other end side of the folded spinal portion, the pressing roller 14 may strongly hit a side edge of the folded spinal portion and may damage (e.g., curl) an edge portion of the sheet bundle SA.
Moreover, when the pressing roller 14 moves along a folded spinal portion while pressing a cover sheet of the sheet bundle SA, the cover sheet may slightly move together with the pressing roller 14. Therefore, the cover sheet may slide relative to inner sheets and may be torn from a binding portion. This problem specifically arises in a case where the cover sheet is thin and the number of sheets is large.